Sodium mordenite separation of para-xylene

ABSTRACT

An improved separation of para-xylene from a mixture of parameta-xylene is achieved by passing the mixture through an adsorption zone of aluminosilicate with a crystalline structure of sodium mordenite for a relatively short adsorption period. The para-xylene is preferentially sorbed.

nited States Patent Chen et al. p 1 June 6, 1972 SODIUM MORDENITE SEPARATION OF R f ren Cited PARA-XYLENE UNITED STATES PATENTS [72] Inven ors: Nai Y en n, h y H nl y J- 3,524,895 8/1970 Chen 260/677 L k R m both of N 3,114,782 12/1963 Fleck et a1. ..260/674 SA Assignec: Corporation Fleck et I 3,126,425 3/1964 Eberly, Jr. et a1 ..260/674 SA 1 Flled= 26, 1969 3,078,643 2/1963 Milton ..260/674 SA [21] App]. No.: 877,577

- Primary Examiner-Delbert E. Gantz Related U.S. Application Data Assistant Examiner-Veronica OKeefe [60] Continuation-impart of Ser. No. 340,789, Jan. 28, Am"eyoswald Hayes and Dnald Dckemn 1964, abandoned, which is a division of Ser. No. 838,023, June 19, 1969, Pat. No. 3,524,895. [57] ABSTRACT An improved separation of para-xylene from a mixture of [52] U.S- Cl ..260/674 parzbmetaxsllene is achieved by passing the mixture through [51] lllt- 1 an adsorption zone of aluminosilicate a crystalline Struc [58] Field of Search ..260/674 SA, 677

ture of sodium mordenite for a relatively short adsorption period. The para-xylene is preferentially sorbed.

1 Claims, No Drawings SODIUM MORDENITE SEPARATION OF PARA-XYLENE CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 340,789, filed Jan. 28, 1964 now abandoned.

This is a division of application Ser. No. 838,023, filed June 19, 1969 now US. Pat. No. 3,524,895 issued Aug. 18, 1970.

BACKGROUND OF INVENTION 1. Field of the Invention.

The art to which the invention pertains relates to the separation of trans hydrocarbon isomers from a mixture thereof with cis hydrocarbon isomers by means of adsorption with crystalline aluminosilicates using relatively short periods of time for adsorption. The invention is particularly applicable to separation of trans olefins and diolefins from mixtures thereof with cis olefins and diolefins.

2. Description of the Prior Art.

US. Pat. No. 2,850,549 issued Sept. 2, 1958, discloses the separation of cis and trans olefin isomers wherein the separation is carried out over a long period of time, e.g. 24 hours.

US. Pat. No. 2,866,835 issued Dec. 30, 1958, is cumulative to US. Pat. No. 2,850,549 in showing that cis and trans isomerscan be separated by means of a crystalline aluminosilicate.

Compared to the prior art, the present invention provides an improved method for obtaining higher yields of trans isomers from mixtures thereof with cis isomers.

SUMMARY OF INVENTION The present invention provides an improved method for separating trans isomers from mixtures thereof with cis isomers. The separation is achieved due to the difference in rate of diffusion of the isomers into the sorption area of the aluminosilicate. When the time allowed for adsorption is relatively short, the trans isomer diffuses more rapidly and greater concentrates or enrichments thereof can be obtained. At equilibrium, the cis isomer is more strongly adsorbed and lesser concentrates of the trans isomer are obtained. To obtain high concentrates of the trans isomer, the adsorption is carried out for a period of time such that adsorption of the cis isomer does not exceed 50 percent of the sorption capacity of the alumonosilicate.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention generally relates to the separation of hydrocarbon isomers. More specifically, the invention is directed to the separation of trans olefins and cis olefin isomers.

The geometric isomers of olefins of four, five, six or seven carbon atoms, e.g., butene, trans-butene-2 and cis-butene-Z, exhibit different physical and chemical properties which make it highly desirable that methods be available for separating them from their mixtures. For example, it has been found that a higher copolymerization rate is realized between ethylene and cis-butene-Z than with trans-butene-Z and that ethylene and cis-butene-2 alternating copolymers are crystalline while the ethylene and trans-butene-2 alternating copolymers are amorphous. Accordingly it is important to be able to separate the isomers of various hydrocarbons.

in general, the present invention comprises separating trans olefins from cis olefins by passing a mixture thereof over a crystalline aluminosilicate adsorbent. The crystalline alumino silicates have been found to preferentially adsorb reliable volumes of trans and cis isomers so that the adsorption of the trans isomer is favored. As an example, calcium zeolite A, no matter how long exposed, will only take up about 20 ml of trans-butene-2 per 100 grams of adsorbent, but will take up most of this in the first minutes of exposure, whereas it takes in cis-butene-2 more slowly.

It has been discovered also that the relative rate of adsorption of trans olefins as compared with the rate of adsorption of cis olefins can be greatly enhanced by causing up to 10 percent and preferably about 5 percent by weight of water or ammonia to be adsorbed in the crystalline aluminosilicate prior to contact with the mixture of isomers.

The crystalline aluminosilicates used for purposes of the invention are known materials and may be described as a threedimensional framework of SiO and A10 tetrahedra in which the tetrahedra are cross linked by the sharing of oxygen atoms whereby the ratio of total aluminum and silicon atoms to oxygen atoms is 1:2. In their hydrated form, the aluminosilicates may be represented by the formula:

wherein M represents at least one cation which balances the electrovalence of the tetrahedra, n represents the valence of the cation, w the moles of SiO and Y the moles of H 0. The cation can be any or more of a number of metal ions, depending upon whether the aluminosilicate is synthesized or occurs naturally.

Typical cations include sodium, lithium, hydrogen, potassium, silver, magnesium, calcium, zinc, barium, iron, nickel, cobalt and manganese. Although the proportions of inorganic oxides in the silicates and their spatial arrangements may vary affecting distinct properties in the aluminosilicate, the main characteristic of these materials is their ability to undergo dehydration without substantially affecting the SiO, and A10 framework.

Among the aluminosilicates which can be employed are sythesized aluminosilicates such as zeolites A and T and natural aluminosilicates such as chabazite, gmelinite, levynite, erionite, offretite, and the like. These aluminosilicates certain an eight-membered ring structure and have a pore size of less than about 5 Angstrom units.

Examples of hydrocarbons which are separated from one another according to the invention are molecules which contain four, five, six or seven carbon atoms per molecule. The invention is particularly applicable to the separation of cis and trans olefins and diolefins such as trans-butene-2 and cis-bu tene-2, trans-pentene-2 and cis-pentene-2, trans-hexene-Z and cis-hexene-Z, trans-hexene-3 and cis-hexene-3, trans-heptene- 3 and cis-heptene-3, as well as diolefins such as mixtures of trans and cis-1,3 pentadiene.

In general, the separation of cis and trans olefins is accomplished by passing a mixture of the isomers through a body or packed bed of the aluminosilicate adsorbent at any temperature at which the mixture can be vaporized without decomposition. The olefin feed may consist of the isomers or an admixture of the same with an inert entraining agent such as nitrogen or helium.

The time period for adsorption is important since it has been found that at short contact times the trans isomers diffuse more rapidly and greater concentrates or enrichments thereof can be obtained. The cis and trans isomers of olefins having four, five, six or seven carbon atoms per molecule are adsorbed by the above-noted aluminosilicates to an extent of less than about 20 percent by weight in accordance with the formula HP 5 0.5 where P is the pressure of the isomer being absorbed, and P is the vapor pressure of the isomers at the temperature of adsorption. Zeolite A, for example, has an adsorption capacity of about 20 weight percent whereas gmelinite, chabazite and levynite have a sorption capacity of approximately 15 weight percent. Aluminosilicates such as Zeolite T, offretite and erionite have a sorption capacity of about 6 to 8 weight percent of the isomers. Accordingly, if the period of time allowed for adsorption is such that adsorption of the cis isomer by the aluminosilicate does not exceed 50 percent of the sorption capacity of the aluminosilicate, and preferably not more than 25 percent, high concentrates or enrichment ratios of the trans isomer can be obtained. As a general guide, the sorption of the cis isomer to an extent greater than 50 percent of the sorption capacity of the aluminosilicates requires at least about 1 hour. Consequently time of adsorption is less than about 60 minutes and preferably less than 10 minutes.

It has been also discovered that the ratio of the volume of the trans isomer relative to the volume of the cis isomer adsorbed at the same pressure, referred to as the enrichment ratio, may be unexpectedly and very significantly increased by the preadsorption of water or ammonia in the aluminosilicate in amounts ranging from 1 to 10 ml per 100 grams of adsorbent.

The following table contains data concerning volume of each isomer adsorbed under the same conditions of vapor and pressure according to Examples 1 through 4.

TABLE 1 Example No. l 2 3 4 Time allowed for adsorption, Minv 1 IO 30 60 Enrichment ratio, trans/cis* 7.9 3.4 2.5 2.0

Ratio of volumes. at normal liquid density, ofadsorhates per 100 gm. dehydrated calcium zeolite A at 26 and 800 mm Hg.

Approximately 92 percent of the total adsorption capacity of calcium zeolite A for trans-butene-2 is taken up in the first 10 minutes. Only 26 percent of the total adsorption capacity for cis-butene-2 is taken up in that time. Therefore, by operating at relatively short adsorption times, preferably 10 minutes or less, better separation of the isomers can be realized due to the difference in the relative volumes of cis and trans isomers that are adsorbed. This is attested to by the much higher enrichment ratios obtained when the time allowed for adsorption is short, as in Example 1. The concentration of the two materials in the atmosphere to which the adsorbent is exposed has little to do with the enrichment rate so long as that atmosphere is not so poor in one constituent as to render the operation pointless.

EXAMPLE 5 The method of Example 1 is repeated except that 1 ml of water is adsorbed per 100 grams of dehydrated calcium zeolite A prior to contact with the butene feed.

EXAMPLE 6 The method of Example I is repeated except that 5 ml of water is adsorbed per 100 grams of dehydrated calcium zeolite A prior to contact with the butene feed.

EXAMPLE 7 The method of Example l is repeated except that 10 ml of water is adsorbed per 100 grams of dehydrated calcium zeolite A prior to contact with the butene feed.

The data resulting from the separation procedures of Examples 5, 6 and 7 conducted over calcium zeolite A containing pre-adsorbed water appear in the following table.

TABLE 2 Example No. 5 6 7 Time allowed for adsorption, Min. 1 l l Ml. H O adsorbed per 100 gm.

calcium zeolite A 1 5 l0 Enrichment ratio, trans/cis 8.4 30.0 6.0

Ratio of volumes. at normal liquid density, of adsorbates per 100 gm. calcium zeolite A at 26 C. and 800 mm Hg.

The data of Table 2 very strikingly illustrates the tremendous improvement in enrichment ratio that results from preadsorption of water onto the calcium zeolite A. In Example 5, the adsorption of 1 ml of water onto the calcium zeolite A before contact with the butene feed, results in an improvement in the enrichment ratio to 8.4 as compared with 7.9 in Example l where no water was pre-adsorbed.

As important as this improvement is, a much greater increase in enrichment ratios is realized when about 5 ml of water per grams of calcium zeolite A is adsorbed prior to contact, as in Example 6. By proceeding according to Example 6, an enrichment ratio, trans/cis, of 30.0 is obtained which is almost a 400 percent increase in efficiency over the fully dehydrated adsorbent of Example 1.

It should also be observed that the improvement in enrichment ratio does not continue to increase in direct proportion to the increase in the amount of water which is pre-adsorbed on the calcium zeolite A. As seen by the results of Example 7, the enrichment ratio decreases to a value of about 6.0 as the amount of pre-adsorbed water is increased to 10 ml per 100 grams of calcium zeolite A.

Thus, it appears that optimum improvement in the ability of calcium zeolite A to adsorb relatively greater volumes of trans-butene-2 in preference to cis-butene-2 at relatively short adsorption times is realized when the amount of pre-adsorbed water is less than about 10 ml per 100 grams of the adsorbent. As will be seen from the following examples, Examples 8, 9 and 10, however, the adsorption of larger amounts of water improve the enrichment ratio at longer contact times.

EXAMPLE 8 The method of Example 3 is repeated except that 1 ml of water per 100 grams of adsorbent is adsorbed into the calcium zeolite A before contact with the butene feed.

EXAMPLE 9 The method of Example 3 is repeated except that 5 ml of water per 100 grams of adsorbent is adsorbed into the calcium zeolite A before contact with the butene feed.

EXAMPLE 10 The method of Example I is repeated except that 10 ml of water per 100 grams of adsorbent is adsorbed into the calcium zeolite A before contact with the butene feed.

The data derived from Examples 8, 9 and 10 are tabulated in the following Table:

TABLE 3 Example No. 8 9 10 Time allowed for adsorption, Min. 30 3O 30 Ml. H 0 adsorbed per 100 gm. calcium zeolite A l 5 l0 Enrichment ratio, trans/cis 2.6 4.2 6.4

Ratio of volumes, at normal liquid density, of adsorbates per lot) gm calcium zeolite A at 26 C. and 800 mm Hg.

Attempts to improve the enrichment ratio of calcium zeolite A by the pre-adsorption of compounds other than water have shown that not all materials produce similar improvement even though adsorbed in the same amounts and utilized under the same conditions. By way of illustration, the following examples are based on the pre-adsorption of ammonia.

EXAMPLE 1 l The method of Example 1 is repeated except that 1 ml of NH is adsorbed in the calcium zeolite A before contact with the butene feed.

EXAMPLE 12 The method of Example 1 is repeated except that 5 ml of NH is adsorbed in the calcium zeolite A before contact with the butene feed.

EXAMPLE 1 3 The method of Example 1 is repeated except that 10 ml of NH;, is adsorbed in the calcium zeolite A before contact with the butene feed.

EXAMPLE 14 The method of Example 3 is repeated except that 1 ml of NH is adsorbed in the calcium zeolite A before contact with the butene feed.

EXAMPLE 15 The method of Example 3 is repeated except that 5 ml of Nl l is adsorbed in the calcium zeolite A before contact with the butene feed.

EXAMPLE 16 The method of Example 3 is repeated except that ml of NH is adsorbed in the calcium zeolite A before contact with the butene feed.

The results of Examples lll6 utilizing calcium zeolite A containing pro-adsorbed amounts appear in the following tahle:

TABLE 4 Example No. l1 l2 l3 l4 l5 16 Time allowed for adsorption, l l l 30 3O 30 Min. Ml. NH; adsorbed per 100 gm. calcium zeolite A l 5 10 l 5 1O Enrichment ratio, trans/cis* 6.4 3.2 1.7 2.4 1.8 1.1

Ratio of volumes. at normal liquid density, of adsorbates per 100 gm. dehydrated calcium zeolite A at 26 and 800 mm Hg.

As will be seen from the above data,'a lower enrichment ratio is obtained with 1 ml of NH; adsorbed in the calcium zeolite A when the time allowed for adsorption is 1 minute than under the same condition with a fully dehydrated calcium zeolite A. This can be seen by comparing the results of Examples 11 and l.

EXAMPLE 17 EXAMPLE 18 The experiment described in Example 17 was repeated except that natural offretite was used in place of calcium zeolite A. Results are summarized in Table 6.

EXAMPLE 19 The experiment described in Example 17 was repeated ex cept that a synthetic zeolite T (erionite) was used in place of calcium zeolite A.

EXAMPLE 20 A mixture of para-xylene and meta-xylene was passed through a packed bed of synthetic sodium mordenite in similar manner as described in previous examples except that the bed was held at 70 C. Results are summarized in Table 8. The

data show that although under the experimental conditions both isomers are adsorbed by the zeolite, it is clearly established from the exit gas analysis that para-xylene is preferentially sorbed, resulting in an enrichment of the metaisomer in the gas stream, and an enrichment of the paraisomer in the adsorbed phase.

Hydro- Hydro- Gas composition, wt. percent Amount sorbed mgJg.

carbon carbon] Time cone, sorbent allowed Inlet stream Exit stream Enrichvol. ratio, for ment percent mgJg. sorption '1. C. 'l./C T. C. T./C. T. 0. ratio 4. 5 11. 3 1. 2 71. 2 20. 4 3. 5 18.1 50.7 36 7. 6 1.1 6. 9 Table 5 2 62 1. 2 59. 3 40. 7 1. 5 40. 0 60.0 67 23 .05 4. 6 1. 0 1. 2 3. 6 41. 0 59. 0 70 15. 4 84. 6 18 46 1. 0 2. 3 5. 7 1. 1 71.2 20. 4 3. 5 49. 2 30. 9 1. 6 3. 2 56 5. 7 Table 6 1. 0 5. 6 1.1 59. 3 40. 7 1. 5 52. 9 47.1 1. 1 1. 8 94 1. 9 1.4 4.6 1.1 41.0 59.0 .7 38.7 61.3 .6 .87 1.1 .8 .0 2.3 1.8 71.2 20.4 3.5 45.3 29.2 1.6 1.3 .24 5.4 Table 7 .8 2.3 1.8 50.3 40.7 1.5 58.1 41.9 1.4 .44 .26 1.7 .8 2.3 1.8 41.0 59.0 .7 32.6 67.4 .5 .56 .57 1.0

Pam Meta P./M. Para Meta P./M. Para Meta P./M.

Tum x 2. a 2.11 a. 3 42.4 57. 0 .74 14. 6 s5. 4 .17 1. 2 1. a .so 3. 8 4. x 3.3 71. 3 28. 7 2.5 53. 2 46. 8 1. 1 3. 2 1. 2 7

Also, by comparmg the results of Examples 1 l, 12 and 13, it We claim:

is seen that increasing the amount of pre-adsorbed ammonia to 5 ml and 10 ml causes a further decrease in the enrichment ratio when operating at an adsorption time of 1 minute.

1. A method for separating para-xylene from a mixture comprising paraand meta-xylenes which comprises:

contacting said mixture with a crystalline aluminosilicate having the crystallographic structure of sodium mordenite for a period of time such that adsorption of the meta-isomer does not exceed 50 percent of the sorption capacity of the aluminosilicate whereby para-xylene is preferentially sorbed, and

separating the unadsorbed phase of said mixture from said aluminosilicate. 

